Method and Apparatus for Managing and Configuring Tracker Components for Enhanced Sensitivity Tracking of GNSS Signals

ABSTRACT

The present invention is related to location positioning systems, and more particularly, to a method and apparatus for making accuracy improvements to a GPS receiver&#39;s navigation calculations. According to a first aspect, the invention provides an extreme sensitivity GNSS tracking architecture. According to other aspects, the architecture includes multiple loops per channel, with the loops implemented with hardware and/or software. According to still further aspects, the architecture includes a multi-level lock detection algorithm designed to provide a trade-off between sensitivity and speed that is not possible with existing tracking architectures.

FIELD OF THE INVENTION

The present invention is related to location positioning systems, and more particularly, to a method and apparatus for managing and configuring tracking components for enhanced sensitivity tracking of GNSS signals.

BACKGROUND OF THE INVENTION

With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. Those systems with full global coverage are sometimes referred to as Global Navigation Satellite Systems (“GNSS”). SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS and/or GNSS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”), any future Western European SPS such as the proposed “Galileo” program, or any other existing or future system such as the Chinese “Beidou” or “Compass” system. As an example, the U.S. NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien New York, 2001, which is fully incorporated herein by reference.

The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.

In environments where satellite signals are degraded, however, a GPS receiver often encounters problems in locking onto the signals that are needed for the calculation of position, velocity, altitude, and time. In a degraded signal environment, satellite signals can be weak or otherwise difficult for GPS receivers to lock on to. One example of a degraded signal environment is a tunnel through which a car equipped with a GPS receiver is driving. While the GPS receiver is in the tunnel, the satellite signals are completely obstructed and no lock can be acquired. As a result, the GPS receiver cannot accurately calculate its position and other navigational data (such as velocity, altitude, and time) while the car is in the tunnel. In addition, because GPS receivers typically utilize positioning algorithms that use navigational filters that account for the GPS receivers' last-known positions, the GPS receiver's position calculation when the car emerges from the tunnel can also be less accurate since accurate position calculations were not performed during the time that the car was in the tunnel.

Degraded signal environments are often encountered in urban areas, such as cities with many tall buildings. A city with many tall buildings can contain “urban canyons”, which are environments where streets cut through dense blocks of structures such as skyscrapers. In urban canyons, satellite signals are frequently not visible or are degraded due to the signals being partially or fully blocked by buildings, for example. Consequently, the problem of inaccurate position calculations by GPS receivers in degraded signal environments is especially acute in urban areas. Other degraded environments include tunnels, garages, and inside buildings.

Existing techniques such as data aiding have succeeded in improving tracking performance in weak signal environments from 30 dB-Hz down to about 10 dB-Hz. However, further improvements in sensitivity are needed, as well as a need to provide a robust architecture that can handle various signal environments and user dynamics.

SUMMARY OF THE INVENTION

The present invention is related to location positioning systems, and more particularly, to a method and apparatus for making accuracy improvements to a GPS receiver's navigation calculations. According to a first aspect, the invention provides an extreme sensitivity GNSS tracking architecture. According to other aspects, the architecture includes multiple loops per channel, with the loops implemented with hardware and/or software. According to still further aspects, the architecture includes a multi-level lock detection algorithm designed to provide a trade-off between sensitivity and speed that is not possible with existing tracking architectures.

In furtherance of these and other aspects, a GNSS tracker apparatus according to embodiments of the invention includes a fast lock detection that calculates a plurality of fast lock detect values for a corresponding plurality of first signal levels; a slow lock detection that calculates a plurality of slow lock detect values for the corresponding plurality of second signal levels; and a threshold detector that compares the calculated fast and slow lock values to associated thresholds.

In additional furtherance of these and other aspects, a GNSS tracker apparatus according to embodiments of the invention includes a first tracking loop for a signal being tracked, the first tracking loop using a first bandwidth; and a second tracking loop for the same signal being tracked, the second tracking loop using a second bandwidth different than the first bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:

FIG. 1 is a block diagram of an example implementation of principles of the invention;

FIG. 2 is a block diagram illustrating an example embodiment of an enhanced sensitivity tracking architecture in accordance with aspects of the invention;

FIG. 3 is a block diagram illustrating one example configuration of a multiple loop per channel tracking architecture according to aspects of the invention;

FIG. 4 is a block diagram illustrating another example configuration of a multiple loop per channel tracking architecture according to aspects of the invention;

FIG. 5 is a block diagram illustrating another example configuration of a multiple loop per channel tracking architecture according to aspects of the invention; and

FIG. 6 is a flowchart illustrating an example multi-level lock detect method according to embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.

FIG. 1 illustrates an example implementation of embodiments of the invention. As shown in FIG. 1, GNSS system 100 includes GPS satellites (i.e. SVs) 114, 116, 118 and 120 that broadcast signals 106, 108, 110 and 112, respectively, that are received by receiver 122 in handset 102, which is located at a user position somewhere relatively near the surface 104 of earth.

Handset 102 can be a personal navigation device (PND, e.g. from Garmin, TomTom, etc.) or it can be a cellular or other type of telephone with built-in GPS functionality (e.g. iPhone, Blackberry, Android smartphone, etc.), or any GPS device embedded in tracking applications (e.g. automotive tracking from Trimble, package or fleet management tracking from FedEx, child locator tracking applications etc).

Receiver 122 can be implemented using any combination of hardware and/or software, including GPS chipsets such as SiRFstarIII GSD3tw, SiRFstar IV, SiRFstar V or SiRFstar GSC3e from SiRF Technology and BCM4750 from Broadcom Corp., as adapted and/or supplemented with functionality in accordance with the present invention, and described in more detail herein. More particularly, those skilled in the art will be able to understand how to implement the present invention by adapting and/or supplementing such chipsets and/or software with the tracking loop improvement techniques of the present invention after being taught by the present specification.

The present inventors recognize that the need for accurate location assistance during situations (e.g. city driving, indoors) where signal levels are very weak (e.g. as low as −165 dBm) is an especially acute need. In general, the invention provides a GNSS tracking architecture that is suitable for performance in challenging signal conditions and user dynamics. According to other aspects, the architecture includes multiple loops per channel, with the loops implemented with hardware and/or software. According to still further aspects, the architecture includes a multi-level lock detection algorithm designed to provide a trade-off between sensitivity and speed that is not possible with existing tracking architectures.

FIG. 2 shows an overview of an example −165 dBm capable tracking architecture in accordance with embodiments of the invention. As shown in FIG. 2, it includes first order code tracking loop 202 and a second order carrier AFC tracking loop 204.

Each loop contains a discriminator (code phase 208, carrier frequency 212) that is gain normalized and/or bandwidth-adjusted using a signal magnitude estimate (M_(est)). In embodiments, the loop discriminators and magnitude estimate use 100 msec coherent integrations as their inputs (I100 msecs, Q100 msecs).

The output of the code tracking loop 202 (θ_(code)) provides the feedback to drive the code NCO, situated before the correlation operation. The output of the carrier AFC loop (F_(carrier)) provides the feedback to drive the carrier NCO, also situated before the correlation operation. An alternative software feedback code/carrier NCO is also possible.

In embodiments, the code tracking loop 202 is first order and is frequency aided from the carrier AFC loop, via the carrier to code aiding block 216. In these and other embodiments, carrier AFC loop 204 is second order. Many alternatives are possible.

The magnitude estimate is formed in block 206 from the on-frequency (0 Hz carrier offset) I and Q punctual correlators.

Example variable-bandwidth embodiments of code tracking loop 202, carrier tracking loop 204, magnitude estimate filter 206 and carrier to code aiding block 216 are described in more detail in co-pending U.S. application Ser. No ______ (SRF-P361A), the contents of which are incorporated herein by reference. Alternative software embodiments of feedback code/carrier loops 202/204, as well as configurations with multiple tracking loops per channel, are also presented in more detail below.

As shown in FIG. 2, according to aspects of the present invention, multi-level lock detects (L_(detN)) are formed by lock detect block 208 via the I_(100 msecs), Q_(100 msecs) correlators. N indicates a lock detect sensitive to a particular CNO. According to further aspects, CNO measurement block 210 forms a CNO estimate using the I_(100 msecs), Q_(100 msecs) correlators as described in more detail below. This estimate provides improved sensitivity over previous designs, as will be described in more detail below.

The following describes example embodiments of multi-level lock detect mechanism 208. In general, lock detect determines if the signal level is higher than a given CNO with a sufficiently high resulting probability of lock detection (P_(d), when signal is present) and sufficiently low probability of false alarm (P_(fa), when signal is absent). The multi-level lock detect according to the invention can be used for many purposes including the following:

-   -   To determine whether a measurement from a track can be used in         navigation (the fast lock detects are designed for this         purpose).     -   To determine whether to kill a track or not (the slow lock         detects are designed for this purpose).     -   To switch a filter period (e.g. a K value) within the CNO         calculation so as to speed up CNO measurement response.

FIG. 6 is a flowchart illustrating one example method of performing and using a multi-level lock detect according to embodiments of the invention.

Every detection interval (step S602), the lock detect for each track is performed. In embodiments, detection intervals are 100 msec, the same as the coherent integration period, but other detection periods are possible e.g. every 1 second. The interval of 100 msecs is convenient because it is also the update rate of the loop feedback and lock detect filters in this design. It should be apparent that in a GNSS receiver, there may be several different tracks that are active at any time, and this method can be performed similarly for each track. First in step S604, the fast lock detects are calculated and it is determined whether any fast lock can be detected for this track in step S606. If any fast locks are detected, the track and associated measurements from the track are marked as valid for this interval in step S614, and processing returns to step S602. Otherwise if step S606 determines that no fast locks are detected, measurements from this track are marked as invalid in step S608. However, the track is not immediately killed because it may still be useful for some purposes. Next, in step S610 the slow lock detects are calculated and it is determined whether any slow locks can be detected for this track in step S612. More particularly, the slow lock detect associated with the lowest target CNO (e.g. 5 dB-Hz) should be found for the track. If not, the track is killed in step S616. Otherwise, the track is kept in step S614, albeit with invalid measurements.

For the determinations in steps S608, and S610, four fast and four slow lock detect CNOs are defined in example embodiments of block 208 according to the invention as shown in Table 1 below. In this example embodiment, the slow lock detect periods are twice as long as fast lock detects, due to the requirement for lower P_(fa) and higher P_(d) associated with killing an enhanced sensitivity track.

TABLE 1 Target CNO (dB- Integration Time Mode P_(fa), P_(d) Hz) (secs) Threshold_(N) Fast 0.01, 0.99 5 30 1.073 Fast 0.01, 0.99 7 12 1.115 Fast 0.01, 0.99 10 4 1.200 Fast 0.01, 0.99 15 1 1.420 Slow 0.001, 0.999 5 60 1.073 Slow 0.001, 0.999 7 24 1.115 Slow 0.001, 0.999 10 8 1.200 Slow 0.001, 0.999 15 2 1.420

In embodiments, the lock detections are formed as follows:

L _(det N)=(Σ_(1 . . . N)(SQRT((I_(100msec)(n))²(Q _(100msec)(n))²)))/N×noise_(est 100msecs)

Where N is the number of 100 msec non-coherent integration periods summed into L_(det N) for each lock detect to be performed. In the example embodiment with TABLE 1, N=300 for the fast lock detect with a target CNO of 5 dB-Hz, N=120 for the fast lock detect with a target CNO of 7 dB-Hz, etc. I_(100msec), Q_(100msec) are the 100 msec coherent integration values. noise_(est 100msecs) is the noise channel estimate referred to the 100 msec period. The 100 msec noise channel estimate is related to the 20 msec noise channel estimate as follows:

noise_(est 100 msec)=SQRT(5)×noise_(est 20 msec)

The value of noise_(est 20 msec) can be determined from a non-tracking channel, because a non-tracking channel usually uses a non-standard code that does not correlate with the satellite codes. The noise channel may also use frequency hopping across, for example, +1-10 kHz to spread the impact of any individual narrowband interferer on the noise estimate. The noise channel can also be estimated across many code and carrier frequency bin hypotheses such that it is heavily averaged across a period of, for example, 20 msec. This results in the variance of the noise channel being small compared with the signal channels. The noise channel is essentially not very noisy compared to the track channels so it does not add significant uncertainty to the Ldet computation compared to the signal power estimate for a given satellite.

So, for example, in step S606 the fast lock detect values are formed using N=10, 40, 120 and 300 and each result is compared to the corresponding threshold value for that integration period in the last column of TABLE 1 (.e.g. Threshold_(N=10)=1.420, . . . , respectively). More particularly, after finding the various values of L_(detN) using each of the N 100 msec correlations and noise estimate above, whether any lock detect has been detected is determined from TABLE 1 as follows:

if (L_(det N) > Threshold_(N)) lock detect = true else lock detect = false

The present inventors recognize that there is a trade-off between very low CNO lock detect and measurement reliability. For very low CNOs, the lock detection may take from tens of seconds to minutes. Clearly a user may travel a significant distance during these periods such that the use of measurements from very low CNO satellite signals must be treated with caution, even if the tracks are kept. Accordingly, examples of using measurements from very low CNO satellite signals include the following:

1. If the receiver has not been able to previously determine a GPS position (e.g. to within several hundred meters) but knows it is stationary (e.g. via MEMS sensors) then very low CNO measurements (and correspondingly slow lock detects) may be effectively used. For example, averaged position fixes may be able to find an otherwise lost object that has an associated tracking device.

2. If the receiver does not require real-time position updates (e.g. no requirement for position fix delay with respect to truth of approximately 1 second) the measurement may be used in real-time but the position fix may be flagged as low quality until a time delay of approximately the lock detect period has passed. Another version of this technique is to delay the measurements used in a position fix by approximately the lock detect period.

It should be further noted that it is possible to trade-off P_(fa), P_(d) versus integration time in various alternative embodiments, essentially tuning the probabilities to the required “trust” level of the lock detects. In the example of TABLE 1, for example, the probabilities have been chosen to provide 99% successful measurement usage from the fast lock detect (measurement won't be used if lock detect indicates false), and 0.1% probability of erroneously killing a track based on the slow lock detect. However, the invention is not limited to this set of example probabilities.

To assist in the decision process about what to do with a track that has passed either the slow or fast lock detects, it is useful to also know the strength of the signal. Returning to FIG. 2, and with regard to determining CNO for a given track, for example as performed by block 210 in step S602, the present inventors recognize that one of the challenges of −165 dBm tracking is measuring the CNO with any sort of accuracy. Accordingly, the following describes example techniques for measuring CNO at −165 dBm according to example embodiments of block 210.

In embodiments, block 210 computes the −165 dBm CNO via an SNR measurement, and the SNR measurement uses a filtered version of the prompt signal magnitude. The prompt on-frequency signal magnitude is given as follows:

M _(prompt)(n)=SQRT[(I _(100 msec)(n))²+(Q _(100 msec)(n))²]

Where I_(100msec)(n) and Q_(100 msec)(n) are I and Q coherent integrations across a 100 msec period. Simulations run by the present inventors with 20 msec coherent integration periods at −165 dBm indicate that the CNO measurement cannot be made accurate enough within any reasonable time period, hence 100 msec coherent integration periods are used. The relationship between SNR and CNO is given as follows:

CNO _(est) =SNR _(est)+10 dB

Where SNR_(est) is the ratio of signal to noise in a 10 Hz bandwidth, and CNO_(est) is the carrier to noise ratio in a 1 Hz bandwidth. A new term SNR_(ratio) is formed as follows:

SNR _(ratio)(n)=(512×M _(prompt)(n))/noise_(est 100msecs)

Where, noise_(est 100msecs) is the filtered noise estimate with 100 msec coherent integration. As set forth, previously, the 100 msec noise estimate can be derived from the msecs noise channel estimate as follows:

noise_(est 100 msec)=SQRT(5)×noise_(est 20 msecs)

Where noise_(est20msec) can be determined similarly as described above.

The filtered version of the SNR_(ratio) thus provides the SNR measurement as follows:

SNR _(meas)(n+1)=[(1−α_(SNR meas))×SNR _(meas)(n)]+[α_(SNR meas) ×DC _(gain) ×SNR _(ratio)(n)]

Where DCgain=2¹⁰=1024, α_(SNR meas)=½^(K). (SNR_(meas)(n+1) is the filtered lock detect output. K controls the loop time constant, and therefore the bandwidth of the CNO measurement. The filter is leaky (or progressive) with time varying time constant K. K initially varies between 0 . . . 10, with K being updated on the 2^(K)th coherent integration period (i.e. PDI), K is updated after the filter is run.

The final K value is altered to vary the CNO measurement pull-in speed based on the fast lock detect estimation of probability that the CNO is greater than a given value. In embodiments, for very low CNOs, e.g. 5 dB-Hz, K is set to 10, providing maximum filtering of the CNO measurement. In these and other embodiments, for moderate CNOs, e.g. >10 dB-Hz, a K value of 8 is used.

In embodiments, block 210 computes the CNO_(est) from the calculated SNR_(meas) value via a lookup table. One example of a lookup table is shown in TABLE 2 below. The lookup table consists of SNR_(meas tabke) values determined via Matlab simulation, in which the address offset of the table refers to a corresponding value of CNO_(est). The CNO_(est) varies between 0 and 54.1 dB-Hz in this example.

TABLE 2 SNR_(meas) SNR_(meas) SNR_(meas) SNR_(meas) SNR_(meas) CNO_(est) _(table) _(table) _(table) _(table) _(table) (dB-Hz) 537 538 539 539 540 0.0, 0.1, 0.2, 0.3, 0.4 540 541 541 542 543 0.5, 0.6, 0.7, 0.8, 0.9 544 545 545 546 547 1.0, 1.1, 1.2, 1.3, 1.4 547 548 549 550 551 1.5, 1.6, 1.7, 1.8, 1.9 552 552 553 554 555 2.0, 2.1, 2.2, 2.3, 2.4 557 557 558 560 560 2.5, 2.6, 2.7, 2.8, 2.9 562 563 564 565 567 3.0, 3.1, 3.2, 3.3, 3.4 567 569 570 572 573 3.5, 3.6, 3.7, 3.8, 3.9 574 576 577 579 580 4.0, 4.1, 4.2, 4.3, 4.4 582 583 584 586 588 4.5, 4.6, 4.7, 4.8, 4.9 590 592 593 595 597 5.0, 5.1, 5.2, 5.3, 5.4 598 601 603 605 607 5.5, 5.6, 5.7, 5.8, 5.9 609 611 613 615 618 6.0, 6.1, 6.2, 6.3, 6.4 620 623 625 627 630 6.5, 6.6, 6.7, 6.8, 6.9 632 636 638 641 644 7.0, 7.1, 7.2, 7.3, 7.4 646 650 652 655 658 7.5, 7.6, 7.7, 7.8, 7.9 661 665 668 672 675 8.0, 8.1, 8.2, 8.3, 8.4 678 682 687 689 693 8.5, 8.6, 8.7, 8.8, 8.9 698 701 705 709 713 9.0, 9.1, 9.2, 9.3, 9.4 718 722 726 731 736 9.5, 9.6, 9.7, 9.8, 9.9 740 745 750 755 760 10.0, 10.1, 10.2, 10.3, 10.4 765 771 775 781 786 10.5, 10.6, 10.7, 10.8, 10.9 793 798 804 809 815 11.0, 11.1, 11.2, 11.3, 11.4 821 828 834 840 847 11.5, 11.6, 11.7, 11.8, 11.9 854 860 867 874 881 12.0, 12.1, 12.2, 12.3, 12.4 889 896 904 911 919 12.5, 12.6, 12.7, 12.8, 12.9 927 935 942 952 959 13.0, 13.1, 13.2, 13.3, 13.4 968 976 984 994 1004 13.5, 13.6, 13.7, 13.8, 13.9 1012 1022 1031 1041 1050 14.0, 14.1, 14.2, 14.3, 14.4 1060 1071 1081 1090 1102 14.5, 14.6, 14.7, 14.8, 14.9 1112 1123 1133 1145 1155 15.0, 15.1, 15.2, 15.3, 15.4 1166 1178 1191 1203 1215 15.5, 15.6, 15.7, 15.8, 15.9 1227 1240 1252 1265 1277 16.0, 16.1, 16.2, 16.3, 16.4 1290 1304 1317 1330 1344 16.5, 16.6, 16.7, 16.8, 16.9 1359 1373 1387 1401 1416 17.0, 17.1, 17.2, 17.3, 17.4 1431 1447 1461 1477 1493 17.5, 17.6, 17.7, 17.8, 17.9 1508 1525 1541 1557 1574 18.0, 18.1, 18.2, 18.3, 18.4 1591 1608 1625 1643 1661 18.5, 18.6, 18.7, 18.8, 18.9 1678 1697 1715 1734 1753 19.0, 19.1, 19.2, 19.3, 19.4 1772 1791 1810 1830 1851 19.5, 19.6, 19.7, 19.8, 19.9 1872 1892 1913 1933 1956 20.0, 20.1, 20.2, 20.3, 20.4 1976 1999 2020 2043 2065 20.5, 20.6, 20.7, 20.8, 20.9 2089 2111 2136 2160 2183 21.0, 21.1, 21.2, 21.3, 21.4 2208 2233 2258 2282 2308 21.5, 21.6, 21.7, 21.8, 21.9 2334 2361 2387 2413 2440 22.0, 22.1, 22.2, 22.3, 22.4 2468 2496 2523 2552 2580 22.5, 22.6, 22.7, 22.8, 22.9 2610 2639 2670 2699 2730 23.0, 23.1, 23.2, 23.3, 23.4 2761 2792 2824 2856 2889 23.5, 23.6, 23.7, 23.8, 23.9 2921 2953 2988 3021 3057 24.0, 24.1, 24.2, 24.3, 24.4 3091 3126 3161 3197 3235 24.5, 24.6, 24.7, 24.8, 24.9 3271 3308 3346 3384 3423 25.0, 25.1, 25.2, 25.3, 25.4 3461 3501 3541 3582 3623 25.5, 25.6, 25.7, 25.8, 25.9 3664 3705 3748 3791 3835 26.0, 26.1, 26.2, 26.3, 26.4 3878 3922 3967 4012 4059 26.5, 26.6, 26.7, 26.8, 26.9 4106 4153 4201 4249 4297 27.0, 27.1, 27.2, 27.3, 27.4 4348 4397 4447 4498 4551 27.5, 27.6, 27.7, 27.8, 27.9 4602 4654 4708 4763 4817 28.0, 28.1, 28.2, 28.3, 28.4 4872 4929 4985 5042 5100 28.5, 28.6, 28.7, 28.8, 28.9 5159 5219 5279 5340 5401 29.0, 29.1, 29.2, 29.3, 29.4 5463 5526 5590 5654 5718 29.5, 29.6, 29.7, 29.8, 29.9 5785 5852 5920 5988 6056 30.0, 30.1, 30.2, 30.3, 30.4 6127 6197 6268 6340 6414 30.5, 30.6, 30.7, 30.8, 30.9 6487 6563 6638 6715 6792 31.0, 31.1, 31.2, 31.3, 31.4 6870 6950 7030 7110 7193 31.5, 31.6, 31.7, 31.8, 31.9 7276 7360 7445 7531 7618 32.0, 32.1, 32.2, 32.3, 32.4 7706 7795 7885 7977 8068 32.5, 32.6, 32.7, 32.8, 32.9 8161 8256 8351 8447 8545 33.0, 33.1, 33.2, 33.3, 33.4 8643 8744 8845 8947 9051 33.5, 33.6, 33.7, 33.8, 33.9 9155 9260 9368 9476 9586 34.0, 34.1, 34.2, 34.3, 34.4 9696 9808 9921 10037 10152 34.5, 34.6, 34.7, 34.8, 34.9 10270 10388 10508 10630 10753 35.0, 35.1, 35.2, 35.3, 35.4 10878 11004 11130 11259 11390 35.5, 35.6, 35.7, 35.8, 35.9 11520 11654 11789 11925 12064 36.0, 36.1, 36.2, 36.3, 36.4 12202 12344 12487 12631 12778 36.5, 36.6, 36.7, 36.8, 36.9 12925 13074 13226 13380 13534 37.0, 37.1, 37.2, 37.3, 37.4 13691 13848 14009 14170 14335 37.5, 37.6, 37.7, 37.8, 37.9 14500 14668 14838 15010 15183 38.0, 38.1, 38.2, 38.3, 38.4 15359 15537 15717 15899 16083 38.5, 38.6, 38.7, 38.8, 38.9 16269 16457 16647 16840 17035 39.0, 39.1, 39.2, 39.3, 39.4 17232 17431 17633 17837 18044 39.5, 39.6, 39.7, 39.8, 39.9 18252 18464 18677 18893 19113 40.0, 40.1, 40.2, 40.3, 40.4 19334 19557 19784 20013 20244 40.5, 40.6, 40.7, 40.8, 40.9 20478 20715 20956 21198 21443 41.0, 41.1, 41.2, 41.3, 41.4 21691 21942 22197 22454 22714 41.5, 41.6, 41.7, 41.8, 41.9 22976 23242 23511 23783 24059 42.0, 42.1, 42.2, 42.3, 42.4 24338 24619 24903 25192 25484 42.5, 42.6, 42.7, 42.8, 42.9 25779 26077 26380 26685 26993 43.0, 43.1, 43.2, 43.3, 43.4 27306 27622 27942 28266 28593 43.5, 43.6, 43.7, 43.8, 43.9 28923 29258 29597 29940 30286 44.0, 44.1, 44.2, 44.3, 44.4 30637 30992 31351 31713 32080 44.5, 44.6, 44.7, 44.8, 44.9 32452 32828 33208 33593 33982 45.0, 45.1, 45.2, 45.3, 45.4 34374 34773 35175 35583 35995 45.5, 45.6, 45.7, 45.8, 45.9 36412 36833 37259 37691 38128 46.0, 46.1, 46.2, 46.3, 46.4 38568 39016 39467 39923 40387 46.5, 46.6, 46.7, 46.8, 46.9 40853 41326 41805 42289 42779 47.0, 47.1, 47.2, 47.3, 47.4 43274 43775 44282 44794 45314 47.5, 47.6, 47.7, 47.8, 47.9 45838 46369 46906 47449 47998 48.0, 48.1, 48.2, 48.3, 48.4 48554 49117 49685 50260 50842 48.5, 48.6, 48.7, 48.8, 48.9 51431 52027 52629 53238 53854 49.0, 49.1, 49.2, 49.3, 49.4 54478 55109 55748 56393 57045 49.5, 49.6, 49.7, 49.8, 49.9 57706 58374 59049 59734 60425 50.0, 50.1, 50.2, 50.3, 50.4 61125 61833 62549 63274 64006 50.5, 50.6, 50.7, 50.8, 50.9 64747 65497 66255 67022 67798 51.0, 51.1, 51.2, 51.3, 51.4 68583 69377 70181 70993 71816 51.5, 51.6, 51.7, 51.8, 51.9 72648 73488 74339 75200 76070 52.0, 52.1, 52.2, 52.3, 52.4 76951 77842 78744 79655 80578 52.5, 52.6, 52.7, 52.8, 52.9 81511 82455 83409 84375 85352 53.0, 53.1, 53.2, 53.3, 53.4 86340 87340 88351 89375 90410 53.5, 53.6, 53.7, 53.8, 53.9 91457 92516 54.0, 54.1, 54.2, 54.3, 54.4

In embodiments, the CNO_(est) lookup involves a binary search of the above table as follows:

min = 0 max = 542 while (max != min) mid = (max + min) / 2 if ( SNR_(meas) (n+1) < SNR_(meas) _(table) (mid)) max = mid − 1 else min = mid end ∥ if statement end ∥ while statement

At the end of this process, the appropriate SNR_(meas table) entry in the table has been identified (i.e. SNR_(meas table) (max=min)), and the corresponding value in the CNO_(est) column is obtained. (Note that for ease of illustration, the table above has been constructed with five SNR entries in separate columns per row, with five corresponding CNO values in the CNO_(est) column).

The present inventors have estimated the expected accuracy of the CNO measurement for several different K values, assuming the filter has reached steady state. For 5 dB-Hz and K=10 the approximate accuracy of the CNO has a standard deviation of about 0.4 dB. For 10 dB-Hz and K=8 the accuracy of the CNO is also approximately 0.4 dB.

The present inventors have also estimated the pull-in performance of the CNO measurement for 5 dB-Hz, K=10 and 10 dB-Hz, K=8 respectively, with an initial SNR_(meas)=0 in both cases. Note that in practice the initial SNR_(meas) may be set to the expected value (derived via lock detect estimate) and K can be initialized to something other than 0 (for smoother CNO measurement pull-in). At 5 dB-Hz the CNO settling time is on the order of about 60 seconds, whereas at 10 dB-Hz the settling time is around 10 seconds.

As mentioned above, the CNO measurement's effective bandwidth is adjusted via the fast lock detects. This mechanism is provided to speed up the CNO response for stronger CNOs. In one example embodiment, the CNO bandwidth is adjusted via the K value as follows:

if (L_(detN=10) = true ) K = 8 else if ( L_(detN=40) = true ) K = 9 else K = 10 end // if statement end // if statement

As set forth above, according to certain aspects, embodiments of the present invention provide a tracker architecture that includes multiple tracking loops per channel. This architecture allows, for example, for primary tracking loops that do not use data bit stripping and secondary tracking loops that use data bit stripping for enhanced sensitivity. This further allows, for example, loops having different coherent integration periods. Still further, this architecture allows, for example, improved tracking performance by cross-track aiding techniques.

In the following examples, a primary tracking loop uses a fixed wide bandwidth, while the secondary tracking loop uses a variable, and likely narrower, bandwidth (i.e. for enhanced sensitivity tracking according to embodiments of the invention). This is not necessary in all embodiments of the invention. Moreover, it may be possible to have more than two loops per channel, for example a third loop using a very narrow bandwidth. Those skilled in the art will appreciate the various alternative embodiments after being taught by the following examples.

In embodiments, there are four configurations of the primary/secondary tracking loops per channel as follows:

-   -   Primary and secondary track channels each use hardware tracking         loops (e.g. hardware correlators, matched filters, integrators         implemented by an ASIC, etc.). This is the hardware intensive         option.     -   The primary track channel uses hardware tracking loops and the         secondary track channel uses software based (e.g. firmware         running on a CPU or DSP) carrier rotation and code phase         interpolation to perform tracking from the hardware correlations         (i.e. software intensive option).     -   Primary hardware track channel only. This mode eliminates the         secondary track channel and hence the enhanced sensitivity track         capability, and so this option is not described in further         detail.     -   Secondary hardware track channel only. This mode eliminates the         primary track channel, using the secondary track channel as the         primary with data bit stripping and enhanced sensitivity         enabled.

In embodiments with approximately 60 track channels implemented by hardware, one preferred implementation is the hardware intensive option. FIG. 3 shows an example block diagram of the hardware intensive option, with primary tracker 302, secondary tracker 304 and cross-track software 306. In this example, primary tracker 302 uses a conventional fixed-bandwidth tracking loop and 20 msec correlation values, unlike the variable-bandwidth tracking loops described in the co-pending application.

Meanwhile, secondary tracker 304 preferably uses a variable-bandwidth tracking loop as described in the co-pending application, including using 100 msec correlation values from coherent integration block 308, which is implemented in hardware. In this example, the 100 msec correlation values are passed to software block 306. Software block 306 includes secondary channel track algorithms 310 (e.g. code phase and AFC track, multi-level lock detect, CNO measurement, etc.), as described above and in the co-pending application, and the code phase and AFC frequency feedback is closed via the secondary hardware channel code and carrier NCOs.

As further shown in FIG. 3, software block 306 includes cross track aiding algorithm 312. This component is provided to initiate secondary tracking loop parameters from primary tracking loop parameters and vice-versa. For example, cross track aiding algorithm 312 receives operating parameters from primary loop 302 and secondary loop 304 and performs calculations including: Code phase, code phase rate, carrier frequency, carrier frequency phase, and carrier frequency rate. Those skilled in the art will appreciate various ways these calculations can be performed from operating values in the code phase and carrier tracking loops. Moreover, those skilled in the art will appreciate various ways that code and carrier tracking loops can be initialized using these calculated values.

In embodiments, cross track aiding can be applied to initialize either a primary or secondary track, assuming that the other track type is currently tracking. Typical aiding examples include:

-   -   Primary track CNO is high (e.g. 20 dB-Hz) and initializes         secondary track.     -   Secondary track has lost lock (e.g. due to dynamics) and is         re-initialized via primary track.     -   Primary track has lost lock (e.g. due to low CNO) and is         re-initialized via secondary track when the CNO rises again.

FIG. 4 shows an example implementation of the dual tracker software intensive mode, with primary tracker 402, secondary tracker 404 and software block 406. In this configuration the secondary track channel 404 is entirely implemented in software block 406. Software code and carrier block 408 forms the secondary channel 100 msec correlations by software manipulation of 5 msec correlations from the primary hardware channel 402. This manipulation includes interpolation of code phase (e.g. at a 10 Hz rate) to generate secondary track correlations via the primary track correlations, and carrier frequency rotation in software (e.g. at a 200 Hz rate) to generate secondary track carrier frequency offsets from primary carrier frequency offsets. These techniques are dependent on the difference in code phase and carrier frequency between primary and secondary channels remaining within certain boundaries. This is the reason 5 msec correlations are used in one preferred example. Those skilled in the art will appreciate that there are various known techniques by which code phase interpolation and carrier frequency rotation can be performed to form the 100 msec correlations from 5 msec correlation values, and so further details thereof will be omitted here for clarity of the invention.

FIG. 5 shows an example implementation of the secondary track channel only configuration according to embodiments of the invention. As shown, this embodiment is similar to the hardware intensive option, but with the primary tracking loop removed, and no cross-track aiding functionality The secondary tracker 504 can thus be implemented in similar fashion as secondary tracker 304 described above. Likewise, secondary track algorithms can be implemented in software as described in that example.

In embodiments including those described above, the secondary track channel preferably uses data bit stripping, for example using data bit prediction such as that described in U.S. Pat. No. 7,924,220, the contents of which are incorporated herein by reference. Such functionality would be included in secondary track algorithms 310, for example. Further aspects of these embodiments are described in more detail in co-pending application Ser. No. ______ (SRF-P361A), including the ability to perform unknown data bit “free-wheeling.”

Although the present invention has been particularly described with reference to the preferred embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications. 

What is claimed is:
 1. A GNSS tracker apparatus comprising: a fast lock detection that calculates a plurality of fast lock detect values for a corresponding plurality of first signal levels; a slow lock detection that calculates a plurality of slow lock detect values for the corresponding plurality of second signal levels; and a threshold detector that compares the calculated fast and slow lock values to associated thresholds.
 2. The apparatus of claim 1, wherein the first and second signal levels are the same.
 3. The apparatus of claim 1, wherein the calculated fast lock detect values are used to determine whether measurements from a track are valid.
 4. The apparatus of claim 1, wherein the calculated slow lock detect values are used to determine whether to kill a track.
 5. The apparatus of claim 4, wherein the calculated slow lock detect values are used to determine whether to kill the track only if none of the calculated fast lock detect values for the track exceed their associated thresholds.
 6. The apparatus of claim 1, wherein the calculated fast and slow lock detect values are together used to determine whether to keep a track even if measurements from the track are not valid.
 7. The apparatus of claim 1, further comprising: a signal strength estimator that provides a strength measurement for a signal being tracked based on a filtered version of a prompt signal magnitude.
 8. The apparatus of claim 7, further comprising a lookup table that associates a plurality of signal strength measurements with estimated signal strength.
 9. A GNSS tracking method comprising: calculating a plurality of fast lock detect values for a corresponding plurality of first signal levels; calculating a plurality of slow lock detect values for the corresponding plurality of second signal levels; and comparing the calculated fast and slow lock values to associated thresholds.
 10. The method of claim 9, wherein the first and second signal levels are the same.
 11. The method of claim 9, further comprising: determining whether measurements from a track are valid using the calculated fast lock detect values.
 12. The method of claim 9, further comprising: determining whether to kill a track using the calculated slow lock detect values.
 13. The method of claim 12, the step of determining whether to kill the track is performed only if none of the calculated fast lock detect values for the track exceed their associated thresholds.
 14. The method of claim 9, further comprising: determining whether to keep a track even if measurements from the track are not valid using both the calculated fast and slow lock detect values.
 15. The method of claim 9, further comprising: calculating a strength measurement for a signal being tracked based on a filtered version of a prompt signal magnitude.
 16. The method of claim 15, further comprising: determining an estimated signal strength value by using a lookup table that associates a plurality of signal strength measurements with estimated signal strength.
 17. A GNSS tracker apparatus comprising: a first tracking loop for a signal being tracked, the first tracking loop using a first bandwidth; and a second tracking loop for the same signal being tracked, the second tracking loop using a second bandwidth different than the first bandwidth.
 18. The apparatus of claim 17, wherein the second bandwidth is variable based on signal strength.
 19. The apparatus of claim 17, further comprising: a cross-track aiding block that enables the first tracking loop to be initiated using values from the second tracking loop.
 20. A GNSS tracking method, comprising: using a first bandwidth tracking loop to track a signal; and using a second bandwidth tracking loop to track the same signal, the second bandwidth tracking loop using a bandwidth different than the first bandwidth tracking loop. 